Every musician, to a greater or lesser extent, has their amp’s sound nailed down; what you might call “the grit.” We’ve been writing songs, rehearsing with our band in the practice room, playing gigs nonstop… All at high volume. But what happens when we need to get the same performance out of the amplifier, only at reduced power?
Setting the Scene: Why Is Power Reduction Necessary?
One day, you and your band decide it’s time to record an album. Obviously, it’s absurd to try to record with a 100 W tube amp set to the same volume you’re used to at rehearsal or at a gig. It just doesn’t work under studio conditions.
What happens? When you turn up your amp’s volume, you’re not only increasing the output power; the sound also gains color, warmth, presence and tone and, above all, you hear the characteristic odd-order harmonics caused by tube saturation.
Tubes work as a function of temperature. We get the best sound at high temperatures, and they heat up more as we raise the volume. If we turn the volume down, on the other hand, we get NOTHING out of the tubes. So if all we want is a good sound for recording, we need to think of some way to reduce the power so we can still turn the volume up.
What about solid-state amps? Those use operational amplifiers and work “cold.” For that reason, turning the volume down is nowhere near as significant, in terms of sound quality, as it is with tube amps.
Finally, it can be worthwhile to reduce power if we play in a small space with a powerful tube amp and want to turn up the volume to get a good sound out of saturated tubes while not blowing out the audience’s ears.
Playing with the Power Tubes: Switches
In this section I’m going to suggest two quite practical solutions. Basically, these are modifications that will let you reduce your amp’s power without using the typical resistive attenuators, which in some cases can cause irreversible damage. This way, you can turn up the volume and get the amp to deliver the sound you were after.
a) HALF POWER
This is a switch that, on amps with an even number of power tubes (generally a minimum of four), lets you disable half of the output tubes, cutting the amp’s power in half.
If we select the “normal” position, the amp will operate in the standard way, using all of the power tubes. If we flip the selector to the “half” position, half of the tubes are disabled and the power is cut in half.
This is very common, for example, on Mesa Boogie amps. For a case in point, check out the article on the “Roadster” model that I recently reviewed for ISP, and you’ll see that the 100 W/50 W power controls work according to this system.
It’s a fairly inexpensive system, and its price usually doesn’t exceed €20. You do have to consider your amp’s electronic characteristics to check whether they match the model, since you may find, for example, that all four power tubes share the same cathode and the switch isn’t designed for that.
b) TRIODE-PENTODE SWITCHING
Triodes are three-electrode amplifier tubes, while pentodes are five-electrode amplifiers, and their linearity and efficiency are better than what you get with triodes. If we opt to install this switch, we can make the tubes in the output stage behave electronically as triodes, achieving a reduction of around 40% in output power. Be aware, though, that in this case the tone and EQ will be affected: the sound loses brightness and gains warmth. You end up with a thicker, creamier, less crunchy sound.
If we set the switch to “pentode” mode, the amp operates in the standard way; if we switch to “triode,” we move into reduced-power mode.
c) WHAT IF WE COMBINE BOTH?
That’s perfectly possible. In fact, in my opinion, it would be the ultimate solution if we don’t want to use power attenuators. It would offer up to four operating modes: “normal,” “half power,” “triode,” or the combination of “half power” and “triode,” which would result in a reduction of almost 75%, making it perfect for studios, for example.
Of course, you have to keep in mind the limitations and consequences of each method, and when you combine both, those still apply. In other words, you’d need a minimum of four power tubes, and switching to triode mode would alter the EQ.
Adding External Elements: Resistive Power Attenuators
What are they? Power attenuators act like big volume controls, causing part of the amp’s power to be dissipated as heat, thanks to the enormous load resistance they contain and the heat sink. The remaining power is passed through to the speaker. The amount of power we want is set with potentiometers, and they may come with additional features, such as boosters, since attenuation often cuts back EQ parameters and it may be necessary to compensate for them.
These devices are often a source of controversy: some consider them ideal, and others take it for granted that they are the worst solution. Today the debate is still open as to how far they can cause problems or be entirely practical.
How to Use Them
Most of these devices are relatively easy to install: you connect the attenuator to the amp’s output transformer (the speaker output jack), and the attenuator to the cabinet. IMPORTANT: just as cabinets and power amps have an impedance, so do attenuators, and they must match the first two. In some cases, because of dummy load impedances, this rule can be bent, but that is not normally the case. You need to consult the attenuator’s specific manual to find out more about this.
What if the amp is wired directly to the speaker, with no such jack? In that case, you have no choice but to have a technician do the work.
The Most Widely Used Attenuators: Features
There are a large number of good-quality attenuators in different formats (rackmount, internal, external…), and since they generate a great deal of heat, they come with a heat sink (it’s very important that it be a good one): a finned, cooled housing, a heat sink or a fan.
a) THD Hot Plate
This is one of the simplest to install and probably the most widely used of all. You only need to use speaker cable (not a 1/4" instrument jack cable), no batteries are required to operate it, it includes a noise gate (up to 16 dB) to prevent any hum from reaching the speaker, and its operating margin of error is one of the lowest. Be aware, though, that at the highest attenuation levels the EQ can be affected, so you’ll need to reset it using the amp’s preamp controls, an active equalizer and/or the Hot Plate’s tone knob. The connection indicator LEDs stand out.
It also has a sturdy housing and a more than acceptable heat sink. It costs between €349 and €440, is compatible with power amps of up to 185 W RMS (150 W RMS being ideal), has an adjustable line out, and is available in 2, 2.7, 4, 8 and 16 ohm versions.
NOTE: NEVER use it with amps that have a solid-state (transistor) output stage.
b) Marshall Power Brake
This external attenuator is perhaps less versatile, and its compatibility and availability across the amp market are somewhat more complicated. It has two 1/4" jack outputs for speakers, an automatic fan cooling system, a choice of 8 or 16 ohms, and a rotary selector with eleven steps of 3 dB each.
It is specially designed for use with Marshall amps (although it is compatible with other brands), and it is at its best for power reduction when you are driving a high-powered tube amp into distortion.
It costs between €450 and €500.
c) Tube Amp Doctor Silencer
A rather peculiar German attenuator in rackmount format. It features a compensation technology (Natural Tone Technology) that, unlike most attenuators, preserves the original tone with no EQ loss, so the brightness and definition of the sound are not affected. It also has “Bite” boosters, to increase the mids, and “Punch,” to increase presence.
It can attenuate amps of up to 150 W RMS (75 W at 2 ohms), maximum noise reduction is up to 16 dB, in 2 dB steps, with a line output (25 kiloohms and F.A.N.T.A. 600 ohms). It is available in 2, 4, 8 and 16 ohm impedance versions.
This is one of the attenuators that has given the fewest problems, and its price is around €420.
Are They Harmful to Our Amp?
The eternal question. In principle, it is not normal for the proper use of a power attenuator to damage the output transformer or the power tubes. Such failures can be attributed to the use of a resistive load, but there have been cases of destroyed transformers while using a Marshall Power Brake with a Marshall head. Forcing the amp’s power reduction beyond the recommended limit means exceeding that amp’s physical limits.
I have a 50 W amp. Wouldn’t it be better to buy a little 15 W one to get half the volume?
It’s unworkable to play at 50 W at home without your neighbors reporting you for disturbing the peace. Many times we have the idea in mind of buying an amplifier to practice at home. It’s not a bad idea, but you should keep in mind the proportion in which volume changes with respect to power.
To analyze this question, you have to understand the logarithmic relationship between the power generated and the perceived loudness. According to research, it follows this equation:
% loudness = 2log10(P2/P1) x 100
So, considering the 50 W amp example, we would get the following results:
POWER 1 (W)-POWER 2 (W)-% PERCEIVED LOUDNESS
40-50 -94
30-50-86
25-50-81
22-50-78
20-50-76
18-50-74
15-50-70
12-50-65
10-50-62
9-50-60
8-50-56
7-50-55
6-50-53
5-50-50
4-50-47
3-50-43
2-50-38
1-50-31
0.75-50-28
0.5-50-25
0.25-50-20
0.10-50-15
0.05-50-13
0.02-50-10
0.01-50-8
0.005-50-6
0.001-50-4
0.0005-50-3
0.0001-50-2
0.00005-50-1.6
0.00001-50-1
With these results, we see that a 15 W amp would not give us a third of the volume produced by a 50 W one, as many people believe, but 70% of it, no less. Keep in mind, though, that this assumes the amp’s physical characteristics are the same.
Also bear in mind that, when it comes to tubes, the type we’re dealing with is VERY significant. That is, a Class A tube has higher linearity and power efficiency than Class A/B, and lower. So if you think that by buying, say, a 20 W Badcat you’re getting an amp for playing quietly at home, you’re making the biggest mistake of your life. I know them well, and I know I could play one in a medium-sized room without any problem. So be careful, and always try out your gear thoroughly.
Conclusion
It’s obvious that at some point in our lives, if we want to use our amp for our musical tasks, we’re going to be forced to reduce the output stage’s power in some way. As noted, for recording or practicing.
The possibilities are very varied, and to each their own. That said, I think that for recording that isn’t terribly important, and for practice, using digital products could be a good idea. Of course, when we’re going to record an album, we want to hear our own sound, and for that we have no choice but to stick to our guns and adjust the power, or have the amp offer the possibility of reducing power via switching (Mesa Boogie models such as the Express, Stiletto, Road King, Roadster…). Attenuators, switching circuits… Pick your favorite; you’re not short on options.